1. Field of the Invention
This invention relates to a spark timing control system for an engine, such as an automotive internal combustion engine.
2. Description of the Prior Art
Japanese patent publication 59-138773 discloses a spark timing control system for an internal combustion engine. In this system, a spark is generated at a moment concurrent with a target spark timing. The target spark timing is calculated from a basic spark timing, and first and second corrective values. The basic spark timing is generally derived from the engine speed and the engine load. The first corrective value, called a knock corrective value, directly depends on whether or not a knock currently occurs in the engine. The second corrective value, called a learning corrective value, consists of a learning control factor which depends on the engine speed and the engine load, and also on a previous first corrective value.
The system in the above-mentioned Japanese document tends to control the spark timing unacceptably during or immediately after abruptly transitional engine operating conditions. This unacceptable control of the spark timing generally results for the following reason. The control of the spark timing in accordance with the knock corrective value has a response speed lower than the speeds of these abrupt transitions between engine operating conditions. This control is maintained during these transitional engine operating conditions.